To the rest of us, people who have  synesthesia seem like they have superpowers.

Or at least, supersenses. They experience the world with one  sense, like hearing a sound or smelling a scent, and then their brains automatically  layer on a totally different sensation.

One person might associate the name  “Phillip” with a bitter orange taste. Another might associate the  word “math” with the color red. And they’d be right!

Math is red,  English is yellow, and Science is green. Of course, simply having a preference  for which notebook color you use in a class isn’t the same  thing as someone actually sensing the colors of letters, numbers, or words. Even if it feels just as strong and personal.

But it turns out that the human  brain is wired in such a way that everyone has a form of synesthesia. Kinda. So in this episode, we’ll  explore five brain-bending ways all of us have senses that overlap! [♪ INTRO] To help us make decisions, our  brains combine information from all our sensory organs through a  process called multisensory integration.

Like, is that alarm and burning smell  coming from my apartment building? Well, I don’t see smoke…so  it might be down the block? But this integration can get a  bit…messy…because our senses can influence each other in predictable, repeatable ways.

For example, your sense of smell  can affect the taste of your food. These are known as cross-modal interactions, and some researchers consider them to be a weak form of synesthesia that we all share. Others, meanwhile, think it  might be misleading to call these cross-modal interactions synesthesia,  because they’re much less intense, and they’re not necessarily  automatic or simultaneous sensations.

So with all that in mind, let’s  get into our first example: how your sense of hearing  affects your sense of touch. Humans have probably been asking each other, “Does your body do this weird  thing too?” for millennia. But a lot of the formal scientific research was only published in the last few decades.

And a 1998 paper was one of the first  studies that documented a particular crossover between hearing and touch that  they called the parchment-skin illusion. The researchers had 17 adults rub their  hands together in front of a microphone, while listening to the sound of that  hand rubbing through headphones. Sometimes, the participants  heard whatever the mic picked up.

Sometimes, they heard the same sound, but quieter. And sometimes, they heard a  version where the experimenters had adjusted just the high frequency  portion of the sound profile… either cranking it up or down by 15 decibels. Of those 17 participants, 13  reported that high-frequency sounds weirdly made their hands feel dryer.

Then, researchers repeated the  trials with 11 of those subjects, asking them to rate the feeling on a  scale of 0…meaning “rough and moist”... to 10, meaning “smooth and dry.” And despite the actual texture of  their hands not actually changing, these 11 participants consistently  reported that the louder and more high-frequency sounds made their  hands feel more “smooth and dry”. Now, the authors of that study didn’t  really explain why they lumped “smoothness” and “dryness” together as one end of  the scale…but both those adjectives describe paper, and presumably led to the  catchy “parchment-skin illusion” term. As for what’s actually happening here,  the skin of your palms and fingertips is packed full of sensory receptors.

And there are specifically  a lot of mechanoreceptors, which sense movement,  stretch, or small vibrations. But you know what else senses vibrations? The hair-like mechanoreceptors in your inner ear that send signals to your brain so you can hear!

Our brains combine hearing and  touch information in regions like the caudo-medial belt of the auditory cortex. So researchers think that when you  hear and feel similar vibrations at the same time, subtle  changes in what your ear detects can interfere with what you  think your hands are feeling. But touch doesn’t just have a  special relationship with hearing.

Your mouth is full of mechanoreceptors too! They help you feel the softness of a fresh peach, or cereal scraping up the roof of your mouth. I’m looking at you, Cap’n Crunch.

And chefs and sommeliers are onto  something with the concept of mouthfeel, which is basically that a food’s  texture affects your dining experience. Because it turns out, there’s a pretty strong cross-modal interaction between touch and taste. A 2015 paper tested this out by  cooking an utterly luxurious recipe that called for a mixture of glucose  syrup, vegetable oil, and citric acid… with either finely ground  or coarse granulated sugar.

Okay, these snacks are definitely  more “science experiment” than “fancy hors d'oeuvres,” but they did  their job: two basically identical foods, with sweet, sour, and bitter ingredients. The only difference was one  was smooth, and one was rough. And when 38 participants ate  these experimental snacks, they rated the smoother  balls as tasting more sweet, and the rougher balls more sour.

The researchers also ran a second  experiment, where participants ate globs of a lemon curd-like gel that was  served on either a smooth or textured plate. This was to see if just seeing a texture could influence your perception of  sour, bitter, or sweet flavors. Turns out, it didn’t, so this cross-modal link seems to be touch-related  rather than sight-related.

While touch and hearing seem to  combine in the brain’s auditory cortex, scientists think that the  sensory information from touch and taste receptors combine  in the orbitofrontal cortex. So that might be the neurological  root of the crossover, but there could be some  psychological aspects at play, too. For instance, our brains are  really good at learning patterns, like what textures and tastes to expect  from the foods we’re used to eating.

Since you can eat all sorts of sour candies  with those grainy citric acid crystals on top, your brain might automatically  perceive more sourness in rough foods. Or, there's this concept of hedonic  match, which is basically the idea that our brain roughly groups pleasant and  unpleasant experiences separately. Soft and sweet things are often perceived  as enjoyable, while rough, sour, and bitter things are often…though not  always...perceived as more uncomfortable.

So these sensations may be  associated in our brains, and influence our perceptions of reality. What you hear can affect  how your food tastes, too, whether it’s just the ambient  noise of a restaurant, or you’re listening to your favorite playlist. A paper from 2011 had 48 participants  eat different foods like chips, cheese, or cake with their eyes closed.

And they had to rate the intensity of sweetness, saltiness, and whether they liked the food. But they also had headphones on and  had to listen to either no sound, quiet white noise at around 50 decibels,  or loud white noise at around 80 decibels. On average, all the foods tasted  less sweet or salty when there was more background noise, but the participants didn’t report that they enjoyed eating them any less.

And by “any”, I of course mean “by  a statistically significant amount”. Which I took notes on in my red math notebook! One neurological explanation for  this effect might be that when your brain processes all this noise, it could be distracted from  processing the taste information.

So instead of combining these sensory  inputs, you’re splitting your attention between two stimuli, and the  taste feels less intense. But there are also cross-modal  interactions between sounds and taste that can enhance certain flavors. In a 2017 study, 116 participants  described the same kind of chocolate as tasting more or less creamy  and more or less sweet, depending on the kind of  instrumental music they listened to.

If they listened to a flute  slowly play a simple pattern, they were more likely to rate the  chocolate as sweeter and creamier. At least compared to when they  heard the sound of violins haphazardly plucking notes with clashing pitches. Marketing researchers have even  gotten involved and branded this kind of cross-modal  interaction as “sonic seasoning.” Which, to be honest, is a much more catchy term!

We think that sonic seasoning may have to do with that hedonic match  concept I mentioned earlier. More specifically, there’s a cognitive  bias called the halo effect where, in this case, if you experience pleasant  low-frequency sounds with your ears, your brain might extend that  pleasantness to your tastebuds by perceiving more sweetness. Thanks to Brilliant for  supporting this SciShow video!

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That link gives you 20% off an annual  premium Brilliant subscription. And you’ll get the first 30 days for free. This next cross-modal  interaction is a close cousin of what we were just talking about.

Because what you hear can also  affect your sense of smell. In one experiment published in 2010, researchers gave 26 participants  two chemical compounds to smell. One was rose-like and pleasant,  called 2-phenylethanol, and one was sort of like parmesan cheese  and…less pleasant…called 1-butanol.

They were asked to rate how pleasant  they found a certain smell after they listened to one of four different  sounds through headphones: a baby laughing, a jazz drum,  a baby crying, or a scream. The halo effect seemed to be involved here, too, since the more enjoyable  sounds made participants report both smells as more pleasant. And I think we all can sort of relate.

It’s hard to focus on anything good  when you’re listening to a scream. But the other main kind of experiment  that researchers use to study this cross-modal interaction looks at  the congruence of sounds and smells. In other words, what happens  when you listen to sounds that make sense with what you’re  smelling versus something random?   That 2010 paper had another experiment  where they gave 22 participants either potato chip or coffee smells to sniff, while having them listen  to a crunching chip sound, a gulping coffee sound, or white noise.

We also came across a paper that  gave 26 volunteers cinnamon and coffee as the smells, and a  Christmas carol, a coffee commercial, or white noise as the accompanying sounds. These studies, and others like them, have found that perceptions of both  the intensity and the pleasantness of odors increased when they matched  participants’ expectations. Neuroscientists don’t have a strong  sense of what’s actually happening here, but our brains might be doing  some kind of pattern recognition or recalling good memories, like  the sounds and smells of a holiday.

So this specific cross-modal interaction  might depend on your lived experiences. Like if you don’t celebrate Christmas, listening to a carol might not do  anything to your perception of smell! A distinctive odor can be really  evocative, from the warm temptation of a chocolate chip cookie to the  “stay away!” of a surprised skunk.

And we’re sticking with  smell for our final example, because smells can also  affect how we see the world. A study from 1997 was one of the first  to explore this cross-modal link. The research team used five different  test smells, from a sugary caramel to leafy galbanum oil and they mixed  up three different intensities of each.

Then, 38 participants flipped through  over 1500 color chips to pick just one that visually matched what they were smelling. The team found that the general color  family stayed consistent for each odor, like red-yellow hues for caramel. But the weaker versions of each odor  were skewed toward lighter colors, and the stronger ones were  matched to darker colors.

Fast-forwarding to 2023, a different  team of researchers took this experiment to the next level by assigning their  participants an achromatic adjustment task. The study had 24 participants sit at  a computer while one of six scents was diffused into the room:  caramel, cherry, coffee, lemon, and peppermint, plus water as a control. At the same time, they were instructed  to use sliders on a computer program to change the color of a 500-by-500 pixel  square so that it was a neutral gray.

Compared to the color they  picked while smelling water, the squares for all the scent  trials were warmer in tone, with a different hue depending on the scent. Although to be fair, the lemon and caramel could’ve been chalked up  mathematically to randomness. But as an example, participants  smelling cherry generally wound up picking a shade of grey  that was actually very slightly red.

Even the peppermint smell was  red, despite the team’s prior work that found an association between  peppermint and blue-green color. This cross-modal interaction may be  related to that idea of congruence, which this team called cross-modal harmony. Our brains might be trying to  balance all the sensory information they receive by matching what we’re  seeing to what we’re smelling.

And what’s weird is that might be  the case even without us consciously knowing what the smells are…sort of like people with synesthesia experiencing the world! The participants were pretty  bad at naming the odors. At most, one-third of them  could guess the correct scent when presented with 26 possible options.

But because of how they colored  that digital gray square, there’s still probably some  underlying recognition or expectation. You know, I wonder what kind of  cross-modal interactions would happen when smelling our Anubis candle  available at complexly.store? How you perceive the world can be so  subjective, because your brain is doing the hard part automatically and unconsciously… whether you have synesthesia or not!

So even if these cross-modal sensations  aren’t as otherworldly as tasting names, it’s still pretty cool that you  might enjoy a creamy dessert just a little bit more if you have  some soothing music to listen to. [♪ OUTRO]